Impregnating carbonaceous brushes with abrasive material



@@arch @mm A. S. BEMIS Feb. 19, 1935.

IMPREGNATING CARBONACEOUS BRUSHES WITH ABRASIVE MATERIAL Filed Feb. 20,1933 Contnkns deposited obmswe INVENToR rur Bem/s BY RNEY' Patented Feb.191, 1935 UNITED STATES PATENT OFFICE IMPREGNATING CARBONACEOUS BRUSHESWITH ABRASIVE MATERIAL Arthur S. Bemis, St. Marys, Pa., assignor toSpeer Carbon Company, a corporation of Pennsylvana Application February20, 1933, Serial No. 657,618

6 Claims.

This invention relates to dynamo electric brushes and other contacts andparticularly to the impregnating of such articles with an abrasive orpolishing substance.

An object of the invention is to impregnato carbonaceous articles suchas brushes or other contacts with a minutely divided abrasive orpolishing agent preferably uniformly distributed throughout the article.

Another object is to saturate a brush with a liquid that is deccmposableby moderate heat into a solid substance having polishing or abrasivequalities.

Another object is to employ such an abrasive or polishing agent thatwill not eloresce on the surface in the forming process, or subsequentthereto.

Another object is to use as such agent a material that will not attack,discolor or otherwise harm the copper or other metal contactingtherewith.

Another object is to use an abrasive or polishing material that will nothave a deleterious effect on the compound usedA to tamp around theflexible cable in fastening it to a carbon article such as a brush, forexample.

Movable contacts of copper or brass with ordinary carbon or graphitehave long been used in switches and as brushes in dynamo electricmachines.

The sole gure of the drawing illustrates a brush containing theinvention.

The forms of carbon used are ordinary or amorphous carbon, naturalgraphite and electrically produced graphite, such as was originallyreferred to as Acheson graphite either with or Without powdered metaldistributed therethrough. Brushes of carbon have been made of amorphouscarbon in comminuted form with a binder such as pitch and then baked tocause the pitch to bind the materials together. Both ordinary oramorphous carbon and natural graphite usually contain sufficientquantities of ash to act as an abrasive agent to keep the coppercommutator or other contact in clean condition. Additional abrasivematerials have sometimes been added to the brush mix for furtherabrasive eiect. These abrasive materials remain in the brush during thebaking process as the baking temperature is not high enough tovolatilize them.

Electrical graphite has been used for brushes and other contacts byforming the desired shapes, either the brush itself or larger slabs ofcarbon stock from which the desired shape may be later sawed, and thenheating them to a very high temperature. To produce graphite thistemperature must in general be from substantially 2500 C. to 3000 C.

Brushes thus made from electrical graphite are substantially purecarbon, speaking generically, and contain practically no ash as thelatter is volatilized at the excessive graphitizing temperature. Suchbrushes seldom contain more than 0.05 to 0.50 per cent. ash. Thesematerials therefore are practically free of polishing material.

Electrical graphite is an excellent material for making brushes Itexcells all other forms of carbon except for the lack of a polishingmaterial. Various attempts have been made to eliminate this one defectby incorporating an excess amount of abrasive material in the carbonbefore graphitization in the hope that enough would survive the highgraphitizing temperature to furnish the abrasive for the brush. With allmaterials thus far tried this has proven impractical. Other attemptshave been made to add abrasive' by impregnating the brush with boricacid, calcium chloride and other soluble compounds and then drying outthe solvent to leave in the solute.

Boric acid and calcium chloride have high vapor pressures and must bedried slowly after impregnating the brush stock therewith and atcarefully controlled temperatures. By this prior process it takes atleast a month to prepare the brush without causing the abrasive toeforesce on the surface of the brush, which action would deplete theinterior of the impregnating compound.

Also attempts have been made to precipitate an abrasive by soaking aprecipitating agent into a brush previously impregnated with a suitableprecipitatablc compound but this causes precipitation only at thesurface because the precipitated material prevents the reagent fromfurther entering the pores.

Various other attempts have been made to incorporate abrasive materialin a brush of electrical graphite but without practical success as faras I am aware.

I have found an entirely practical way of incorporating a polishing orabrasive material in a brush of electrical graphite in a comparativelybrief period.

The brushes are made up in the desired form of carbonaceous material, ora slab of brush stock is formed, and then heated electrically to thetemperature of graphitization for a sufficient period to convert thecarbon into graphite. The graphite brushes, or graphite stock, as thecase may be, is then impregnated or saturated with a silicon ester suchas ethyl silicate, (C2H4)4Si304. The silicon ester may be readily madeby adding anhydrous ethyl alcohol to silicon tetra-chloride. Theimpregnation is preferably carried on under vacuum with later appliedpressure to saturate the brush or stock throughout with the compound.After impregnation the stock is dried in the air for several days andthen bake-d to 600 C. or even somewhat higher. This transforms the ethylsilicate into silicon dioxide (SiOz) in extremely ne condition in thepores of the graphite and the whole process may be completed in threedays. The quantity of SiOz precipitated in the brush depends among otherthings, upon the concentration of the ethyl silicate in the ethylalcohol. Various other esters may also be used, for example, the estersof tin and titanium. These may be made by adding ethyl alcohol to thechlorides of tin and titanium respectively. All these, and similarlyacting campounds, are regarded as equivalents of silicon ester and theresulting abrasives are equivalents.

The silicon or other oxide formed from the esters is in very minutesize, much finer than the silicon dioxide produced from sodium silicateand consequently it does not mar or scratch the commutator. It merelypolishes the commutator and keeps it bright and clean, though it can becalled an abrasive in the sense that it does Wear the surface of thecommutator enough to keep it smooth. The polishing or abrasive actioncan be controlled 5y varying nt of 'thersilicon dioxide, or stannic ortitanium dioxide, incorporated' in the material by the new process. Ashas been said, a convenient Way to accomplish this is to vary theconcentration of the ester in the ethyl alcohol. In the drawing thebrush 1 contains the evenly distributed abrasive and may have any formof cable desired. I have illustrated the cable 2 as being held in arecess 3 by tamped metal but this forms no part of the invention.

The invention is also not limited to esters or any specific compound, asany liquid compound can be used to saturate the brush if it isdecomposable by heat to an abrasive or polishing solid 0 in situ.

A brush made from the treated stock does not eiiioresce and does notchemically attack the commutator or the material used to tamp thepigtail cable therein.

While I prefer to use electric graphite as the brush stock on account ofits excellent qualities, I may nevertheless, secure some of the resultsof the novel invention by using ordinary carbon, or natural graphite,either mixed with powdered metal or not, and precipitate the abrasivetherein. This would be especially advantageous where the carbon ornatural graphite has been puriiied and its ash content accordinglyreduced.

Various modifications may be devised and the invention is not to belimited to the specific disclosure.

Having described my invention, what I claim l. The process of making abrush for dynamo electric machines which consists in first forming abrush body of carbonaceous material, saturating the body with a liquidcompound decomposable by moderate heat into a polishing or abrasivesolid and heating said body to form said abrasive solid directly withinthe body.

2. 'I'he process of incorporating abrasive material in a brushcontaining carbon which consists in adding ethyl silicate thereto, andforming silicon dioxide therefrom.

3. The process of incorporating polishing ma.- terial in a brushcomposed in part of carbonaceous material which consists in soaking thebrush in ethyl silicate, drying and baking the brush to form silicondioxide therein.

4. The process of making a brush which consists in forming electricalgraphite into the desired shape, soaking the graphite shape with ethylsilicate solution, drying out the solvent and baking to form silicondioxide in minute particles in the pores of the electrical graphite.

5. A brush for dynamo electric machines consisting of a graphitized bodyand a graphitized binder and having minute particles of an insolubleabrasive oxide exceeding 0.5% in amount uniformly distributed throughoutthe graphite material.

6. A brush for dynamo electric machines consisting of a graphitized bodyand a graphitized binder and having minute particles of silicon dioxideexceeding 0.5% in amount uniformly distributed throughout the graphitematerial.

ARTHUR S. BEMIS.

